19... a8a6 Removing the rook from the attack and defending the b6-pawn. Ding has accurately calculated the consequences of White's most direct move (20.Qb5), but there's another possibility. 20. b2b5 This lets any chance of an advantage slip. 20... a6a2  21. c4b6 Apparently, Black has problems with the pinned c-pawn, but he solves all of them with the next move. 21... c7a7 Removing the pin and supporting a timely Ra2-a5.